With the trend towards energy conservation and environmental protection, illuminant modules using light-emitting diodes as light sources have gradually replaced common incandescent lamps and fluorescent tubes. Currently, the light-emitting diode illuminant module is fabricated by soldering surface mounting device (SMD) light-emitting diode package structures onto a circuit board using a surface mount technique. The power is provided to the circuit board and is delivered respectively to p-type electrodes and n-type electrodes of light-emitting diode chips through lead frames of the light-emitting diode package structure to make the light-emitting diode chips emit light.
However, the conventional light-emitting diode chip generally cannot transform all of the input electric energy into light energy, and the electric energy is mostly lost in the form of thermal energy, so that the transform efficiency is poor. If the heat cannot be effectively dissipated, the junction temperature of the light-emitting diode chip is further increased, thereby reducing the luminous efficiency of the light-emitting diode chip and decreasing the reliability of the light-emitting diode chip. Therefore, how to resolve the heat-dissipating problem has become an important subject of the development of the light-emitting diode device.
Refer to FIG. 1. FIG. 1 illustrates a cross-sectional view of a conventional light-emitting diode illuminant module. The conventional light-emitting diode illuminant module 100 is a surface mounting type, high power light-emitting diode illuminant module. The light-emitting diode illuminant module 100 mainly comprises a light-emitting diode package structure 136 and a circuit board 134. The light-emitting diode package structure 136 mainly comprises a light-emitting diode chip 102, a package base 104, an encapsulation layer 126 and a reflective layer 124. The package base 104 includes a lead frame 106 embedded in the package base 104. The lead frame 106 includes two leads 108 and 110 and a metal heat sink 112, wherein the metal heat sink 112 is electrically connected to the lead 110. The package base 104 is formed of polyphthalamide (PPA) by an injection-molding method and includes a cavity 118. The cavity 118 exposes a portion of the metal heat sink 112 and a portion of the lead 108, and the reflective layer 124 is disposed on a side surface of the cavity 118. The light-emitting diode chip 102 is disposed on the exposed portion of the metal heat sink 112 exposed by the cavity 118. One electrode of the light-emitting diode chip 102 is electrically connected to the metal heat sink 112, and the other electrode is electrically connected to the lead 108 via a wire 120. The encapsulation layer 126 is filled into the cavity 118 and covers the light-emitting diode chip 102, the wire 120 and the lead frame 106 exposed by the cavity 118. In order to provide a light of a desired color, the encapsulation layer 126 may be added with fluorescent powders 122.
The circuit board 134 mainly comprises a carrier substrate 128, an insulation layer 130 and a circuit layer 132, wherein the insulation layer 130 is located between the circuit layer 132 and the carrier substrate 128 to electrically isolate the carrier substrate 128 and the circuit layer 132. The circuit board 134 may be a metal core printed circuit board (MCPCB), and the carrier substrate 128 is typically an aluminum substrate. The circuit board 134 may also be a FR4 copper clad laminate. The light-emitting diode package structure 136 is disposed on the circuit board 134. The leads 108 and 110 are electrically connected to the circuit layer 132 of the circuit board 134 respectively via solder bumps 114. The metal heat sink 112 is connected to the carrier substrate 128 of the circuit board 134 via a solder bump 116 to dissipate the heat produced by the light-emitting diode chip 102.
In order to increase the heat-dissipating efficiency of the light-emitting diode illuminant module 100, the light-emitting diode chip 102 of the light emitting diode illuminant module 100 is connected to the metal heat sink 112 of the lead frame 106 to conduct the heat produced by the light-emitting diode chip 102 directly downward to the underlying carrier substrate 128 of the circuit board 134. Therefore, a better heat-dissipating effect of the light-emitting diode illuminant module 100 can be obtained by using the carrier substrate 128 made of the aluminum substrate.
However, the light-emitting diode chip 102 of the conventional light-emitting diode illuminant module 100 needs to be packed on the package base 104 formed of polyphthalamide by a package factory in advance, and then delivered to so called SMT (Surface Mounted Technology) factory to mount the light-emitting diode package structure 136 on the circuit board 134. In addition, the metal heat sink 112 needs to be formed simultaneously with the lead frame 106, so that the irregular and large thickness difference of the lead frame 106 will further complicate the process to fabricate the lead frame 106 and causes a material waste issue. These package and manufacture procedures all increase the complexity of the process of the light-emitting diode illuminant module 100. Furthermore, the light-emitting diode chip 102 is usually connected to the metal heat sink 112 by sliver glue, so that the heat conduction of the light-emitting diode chip 102 is limited by the heat-conducting ability of the silver glue, thereby resulting in a poor heat-dissipating effect. When the circuit board 134 is an FR4 substrate, the heat dissipating effect of the light-emitting diode illuminant module 100 is poor due to low thermal conduction coefficient of the FR4 substrate. When the circuit board 134 is a metal core printed circuit board, the cost of such circuit board 134 is high because the process of the metal core printed circuit board is complicated. Moreover, after performing the anneal process following the surface mounting technology bonding process used to bond the lead frame 106 of the light-emitting diode illuminant structure 136 and the circuit layer 132 of the circuit board 134, a failure of an unevenness of solder bumps 114 between leads 108 and 110 is easily found. As a result, the light-emitting diode package structure 136 is oblique, so that the collinearity of the light-emitting diode package structure 136 is poor, thereby degrading the illuminant uniformity of the light-emitting diode illuminant module 100.